Cell culture of animal cells, in particular mammalian cells, is important for the production of many important (genetically engineered) biological materials such as vaccines, enzymes, hormones and antibodies. The majority of animal cells are anchorage-dependent and require attachment to a surface for their survival and growth.
Routinely, anchorage-dependent cells have been cultivated on the walls of for instance tissue culture flasks and roller bottles. As the necessity has developed to provide large amounts of certain antiviral vaccines, genetically engineered proteins, and other cell-derived products, improvements have been made to develop new systems for larger scale production of cells.
One such an improvement started with the development of microcarriers in 1967 by Van Wezel (Van Wezel, A. L. Nature 216:64-65 (1967)). Van Wezel made micro-carriers composed of cross-linked dextran beads charged with tertiary amine groups (DEAE). He demonstrated the attachment and growth of cells on these positively charged DEAE-dextran beads suspended in culture media in a stirred vessel. Thus, in microcarrier cell cultures cells grow as monolayers on small spheres which are in suspension. By using microcarriers it is possible to achieve yields of several million cells per millilitre. Over the years various types of microcarriers have been developed. For instance glass beads and polystyrene beads have been described. Cross-linked dextran, like the first microcarriers, is still the most popular bead material.
Some advantages of microcarrier cultures over other methods of large-scale cultivation are: i) high surface area to volume ratio can be achieved which can be varied by changing the microcarrier concentration leading to high cell densities per unit volume with a potential for obtaining highly concentrated cell products; ii) cell propagation can be carried out in a single high productivity vessel instead of using many low productivity units, thus achieving a better utilisation and a considerable saving of medium; iii) since the microcarrier culture is well mixed, it is easy to monitor and control different environmental conditions such as pH, pO2, pCO2 etc.; iv) cell sampling is easy; v) since the beads settle down easily, cell harvesting and downstream processing of products is easy; vi) microcarrier cultures can be relatively easily scaled up using conventional equipment like fermenters that have been suitably modified.
When developing further improvements the following requirements for an optimum microcarrier should be met: i) the surface properties of the beads should be such that cells can adhere and proliferate rapidly, preferably the contour should be even; ii) the density of the beads should be slightly more than that of the culture medium, so as to facilitate easy separation; conventional culture media are aqueous in nature and have densities ranging from 1.03-1.09 g/cc, however, the density should not exceed a certain limit the optimum range being 1.03-1.045 g/ml; gentle stirring, which will not harm the shear-sensitive cells, should be sufficient to keep them in suspension, if the beads settle cell growth will be prevented; iii) the size-distribution of the beads should be narrow so that an even suspension of all microcarriers is achieved and cells attain confluency at approximately the same time; also, clustering of microcarriers in solution should be prevented; iv) the optical properties should enable easy microscopic observation; v) they should be non-toxic not only for the survival and good growth of the cells but also for cell culture products that are used for veterinary or clinical purposes; vi) the matrix of the beads should be such that collisions, which occur during stirring of the culture, do not cause fragmentation of the beads.
An important modification in the development of improved microcarriers is the coating of core particles with collagen. The advantage of using collagen is that it is a promoter for both cell attachment and cell growth. In addition cells can be easily detached by proteolytic enzymes. Also microcarriers coated with fibronectin, which is a cell adhesion promoter, have been described.
Cell surface receptors that are involved in binding are identified as integrins. More than 20 integrins are known, each having different ligand specificities. Many integrins can recognise the amino acid sequence RGD (arginine—glycine—aspartic acid). The number of proteins known to comprise an RGD sequence is limited, estimated to be approximately 400. Also not every occurring RGD sequence is involved in a binding function. In particular RGDS is known to be involved in cell adhesion. It is known that natural native collagens comprise the amino acid sequence RGD. Examples are for instance murine COL1A1-1, COL1A1-2, COL1A1-3, rat COL3A1, human COL1 A-1, COL2A-1, COL3A-1 and COL1A1-2. Cell attachment to collagen follows a highly specific mechanism, as cells secrete the protein fibronectin which has a specific affinity for collagen. After attachment to collagen, the fibronectin binds subsequently to the integrin proteins in the cell.
Gelatine is a degradation product of collagen, and the term truly reflects a heterogeneous mixture of proteins and peptides with MW's ranging from 5,000 up to more than 400,000 daltons. A certain fraction of these gelatine polymers will contain an RGD motive and a certain fraction of hydrolysed molecules will not have such an RGD motif.
Burgess and Myles describe the chemical coupling of RGD containing peptides to type I collagen, which method is complicated and introduces impurities into the target materials and has limited modification freedom (Ann Biomed Eng January 2000; 28(1):110-118).
U.S. Pat. No. 5,512,474 aims at a good cell adhesion by combining cell adhesion factors and positively charged molecules to the surface of a cell culture support. Fibronectin is identified as a cell adhesion intermediate.
U.S. Pat. No. 5,514,581 is concerned with the production of substantially new polypeptides by using automated methods for the chemical synthesis of DNA in combination with recombinant DNA technology. The patent describes non-natural polypeptides which may be of use in commercial applications for which known, naturally occurring polypeptides are not appropriate. Although the artificial proteins could contain an RGD sequence, the resulting bio-polymers have little resemblance with naturally occurring proteins, and will cause immunological problems in case of medical applications.
Cell attachment also plays an important role in medical applications such as wound treatment (including artificial skin materials), bone and cartilage (re)growth and implantations and artificial blood vessel materials. Thus in medical applications often the demand is that a material has a biocompatible coating in terms of cell attachment. In general collagen is preferred over structures like those described in U.S. Pat. No. 5,514,581 since collagen has a low antigenicity and is present in both skin and bone or cartilage. Microcarriers coated with gelatine are applied, but also porous matrixes of collagen are used. An example is collagen coated beads to grow keratinocytes and the subsequent transplantation of the beads containing keratinocytes to promote healing of wounds is described in U.S. Pat. No. 5,972,332. Another area of interest in relation to cell attachment is the blocking of attachment receptors of cells. For instance by blocking the attachment receptors cancer metastasis may be influenced or inhibited. Also smaller peptides with RGD sequences are used to block the binding sites on cells thus preventing unwanted interactions like aggregation or coagulation as described for example in EP 0,628,571.
There is however an urgently felt need for further improvements of materials for use in applications involving cell attachment. The fibronectin-mediated cell attachment mechanism is effectuated to attach cells to natural gelatines. However, natural gelatines are not safe in terms of prion and virus impurities. The RGD mediated mechanism for cell attachment is effectuated by several artificial bio-polymers, produced by chemical or by recombinant DNA technologies. However, these materials have non-natural sequences and have a risk of initiating an immunological response which could result in serious medical problems.
Whereas often the terms ‘collagen’, ‘collagen-related’, collagen-derived or the like are also used in the art, the term ‘gelatine’ or ‘gelatine-like’ protein will be used throughout the rest of this description. Natural gelatine is a mixture of individual polymers with MW's ranging from 5,000 up to more than 400,000 daltons. The terms cell adhesion and cell attachment are used interchangeably. Also the terms RGD sequence and RGD motif are used interchangeably.